>>108757
If you had access to a working card, with the right equipment (pc, card reader and writer, spare card), you could program another card with the info on the card you have access to, it's like making a copy of a key.
If you don't want to get caught you'd have to know how advanced the system is, is it a simple door mechanism?, or part of a larger network that keeps track of who accessed what and logs card holders actions, name, time, place, etc.
Some advanced tricks include installing skimmers on card readers, long range rfid readers, etc.
pretentious article, but worth a read
http://www.wired.com/2006/05/rfid-2/
Also the most popular systems right now are magnetic, chips, rfid, and nfc, nfc and rfid don't require any physical contact between the card and the reader and their cards are just blank mostly, chips require being inserted for at least a small window of time and of course have a metal chip on top, magnetic is activated by swiping and the card has a magnetic bar along it's length.